Deep ultraviolet light sources, such as those used for integrated circuit photolithography manufacturing processes have been almost exclusively the province of excimer gas discharge lasers, particularly KrF excimer lasers at around 248 nm and followed by ArF lasers at 198 nm having been brought into production since the early 90's, with molecular fluorine F2 lasers also having also been proposed at around 157 nm, but as yet not brought into production.
To achieve resolution reduction at a fixed wavelength and fixed NA (i.e., an 193 nm XLA 165 on a XT:1400 with an NA of 0.93), one must optimize k1, where k1 represents process-dependent factors affecting resolution.
Based on Rayleigh's equation, for dry ArF tools today smaller resolution of state-of-the-art high-numerical-aperture ArF lithography can only be achieved with Resolution-Enhancement Techniques (RET's). RETs are a cost-effective way to maintain the aggressive evolution to smaller dimensions in IC manufacturing and are becoming integral to manufacturing lithography solutions.
These Process-related resolution enhancement efforts (lowering k1) have focused on reticle design, using methodologies such as phase shifting or pattern splitting on dual masks. While these techniques improve imaging, they also have significant drawbacks, including throughput loss. So when k1 is optimized for an application the only way to improve resolution further is to go back to the wavelength or NA.
Immersion lithography does just this for the 45 nm, the wavelength is constant at 193 nm so introducing water allows for NA's up to 1.35 and this relaxes the k1 requirement until processing at the 32 nm is required.
Since first introduction of excimer laser light sources in the DUV wavelength manufacturers of these light sources have been under constant pressure not only to reduce the wavelength, but also to increase the average power delivered to the wafer in the manufacturing process carried out by the steppers and scanners of the principle customers for such light sources, the stepper/scanner manufacturers, now including Canon and Nicon in Japan and ASML in the Netherlands.
This requirement for smaller and smaller wavelength has come from the need of the integrated circuit manufacturer customers for the stepper/scanner makers to be able to print smaller and smaller critical dimensions on the integrated circuit wafers. The need for higher power has generally been driven by either the need for more throughput or higher dose for exposing certain photo resists on the wafer, or both.
This steady progression down the road formed by the so-called Moore's law about the progression of integrated circuit capabilities, and thus, the number of transistors per unit area and thus also basically smaller and smaller critical dimensions, has created various and serious problems for the light source manufacturers to address. Particularly the move to the 193 nm wavelength node of light sources has resulted in several challenges.
The lower wavelength photons from an 193 nm laser system has having higher energies than the prior KrF 248 nm light sources has caused problems both for the light source manufacturers and the present day scanner manufacturers. Regions in both the light source and scanner receiving these higher energy photons, particularly at high energy density levels per unit area have been required to be made of what is currently the single window/lens material that can stand up to these optically damaging photons for any reasonably economical period of time, i.e., CaF2. Such single material lenses in the scanners have required the scanner manufacturers to, e.g., demand virtually monochromatic light out of the laser light source systems, e.g., to avoid chromatic aberrations in the lenses.
The demand for smaller and smaller bandwidths (more and more monochromatic light) has required more and more precisely sensitive line narrowing units, e.g., containing etalon or grating line narrowing optical elements. Older style single chamber laser light sources suffered from short life of such line narrowing units because, among other things, much of the light entering the line narrowing unit is lost in the line narrowing process, the narrower the output bandwidth being required the larger the loss. Thus, the requirement was to send higher and higher pulse energy into the line narrowing unit in order to get a given pulse energy out of the laser system. Thus, e.g., formerly utilized single chamber ArF laser systems were capable of laser output pulses of around 5 mJ with reasonably cost effective lifetimes for the line narrowing units.
A first approach of the light source manufacturers, among other things, to address these issues with ArF light sources to get progressively narrower bandwidths and higher output average power was to increase pulse repetition rate, with essentially the same pulse energy per pulse. Thus through about 2002 pulse repetition, rates increased from hundreds of pulses per second to 4 kHz. This kept the optical damage per pulse down, but increased the overall exposure of the laser optics in the line narrowing units and elsewhere as the pulse repetition rate increased. In addition, higher pulse repetition rates created other problems for the light source manufacturers, principal of which were, e.g., increased electrode deterioration rates and requirements for faster gas circulation rates within the lasing chamber, i.e., requiring more fan motor power and adding more heat to the chamber and the fan motor and bearing assembly, resulting in reductions in mean time between replacement for the lasing chambers.
Higher pulse repetition rates also caused problems that the light source manufacturers had to address, e.g., in the magnetic switched pulse power supplies with timing and component lifetimes negatively impacted by the higher thermal loads, e.g., on the magnetic switching elements in the pulsed power system at the higher pulse repetition rates.
To add to the difficulties to be addressed by the light source manufacturers the integrated circuit have also continued to demand improvements in other laser pulse parameters, e.g., beam profile and beam divergence and pulse-to-pulse stability requirements, e.g., for bandwidth and energy and timing from the trigger signal from the scanner, etc. The ability to provide the various controls of the laser output and operating parameters of this nature can be negatively impacted by either or both of increased pulse energy demands and higher pulse repetition rate, along with variations in such things as the duty cycle (percentage of time the laser is firing) during operation), pulse energy selected by the scanner, rates of depletion of F2 in the lasing chamber, etc.
For example the applicant's assignee's product the ELS-6010, The world's first variable 248 nm KrF excimer single chamber laser system, introduced in the late nineties, provided what was then advanced optical performance applicable to 130 nm node of semiconductor manufacturing. It provided what was then also a highly line-narrowed bandwidth of about 0.5 pm at full-width half-maximum (FWHM) and about 1.4 pm (95% energy integral), thus enabling lithography steppers and scanners to achieve full imaging performance using lenses with numerical apertures of >0.75. The ELS-6010 supported higher throughput rates for its day, e.g., operating at up to 2.5 kHz, 8 mJ pulse energy, for 20 W average power, the ELS-6010 also delivered what was then improved dose stability at the wafer for better CD control and higher yield. The ELS 6010 also provided for precise energy control to reduce the need for attenuation, optimize pulse usage, and extended the useful lifetime of laser consumables. Improvements to signal processing components in the wavelength stabilization module provided faster data acquisition and more reliable wavelength stability.
The ELS 6XXX models were followed by a later model, called by applicant's assignee the ELS 7000, addressing even more aggressive requirements of the semiconductor industry for the sub-130 design node. This also KrF 248 nm wavelength excimer single chamber laser system delivered even more tightly constricted bandwidth at higher power in order to reduce CD geometry in semiconductor photolithography, further improving throughput, and reducing operating costs. The 7000 was also made available in an ArF 193 nm wavelength version. More average power was delivered by increasing pulse repetition rate from 2.5 kHz to 4 kHz. The 7010 added improved line narrowing performance (selecting bandwidth) and wavelength stabilization, e.g., to insure better focus control, maximize exposure latitude, and improve semiconductor circuit critical dimension (“CD”) control. Improvements were also made in the gas injection algorithm, e.g., for injecting small, precise amounts of gas into the laser chamber during exposure sequences to provide superior energy stability. The ELS-7000 was aimed to meet the requirements of high volume product of sub-0.13 micron devices on 248 nm exposure tools. Offering 4 kHz, 7.5 mJ, 30 W optical output, plus the same ultra-low bandwidth performance as the 6010, and high-speed wavelength control, the ELS-7000 also reduced laser consumables costs.
The ELS 7000 was followed in about 2001 by applicants' assignee's ELS 7010 model that further aggressively addressed the performance and cost requirements of the semiconductor industry for the sub-100 nm design nodes. The ELS-7010, also a 4-kHz krypton fluoride (KrF, 248 nm), excimer light source addressed the demands of the photolithography by the semiconductor industry for sub-100 nm design nodes. The ELS-7010 offered further increased power and bandwidth performance parameters for KrF light sources and still further decreased the cost of consumables (CoC). The ELS-7010 provided a 50 to 100 percent improvement on the expected life of each of the major consumable modules, while at the same time increasing power and further reducing bandwidth. The ELS 7010 was a 4 kHz, 10 mJ, 40 W, (FWHM) 0.35 pm and (E95%) 1.2 pm, single chamber laser system.
Another follow on, and probably the further extent of single chamber excimer laser technology was applicants' assignee's Nanolith 7000 ArF (193 nm) single chamber laser system introduced in about 2002. The Nanolith 7000 was an ArF laser system with about the same bandwidth specifications at 193 nm as the ELS 7000, i.e., ≦0.5 pm FWHM and ≦1.3 pm (E 95% intensity integral) at 5 mJ and 4 kHz (20 Watt) operation to power next-generation lithography tools with superior spectral power and highly focused line-narrowed bandwidth, while again reducing laser consumable costs, a more difficult task at 193 nm, due mostly to increased optical damage resulting from the reduced wavelength light. The 193 nm Nanolith ArF light source for volume photolithography semiconductor manufacturing, with its highly line-narrowed, high power and high accuracy tuned wavelength control enabled the most advanced imaging of that time, e.g., with NA>0.75 below the 100 nm node, still meeting the then current such other requirements as image contrast and wafer throughput, enabling chip design to shrink even further, accommodating, e.g., faster processor speed, larger memory capacity per chip, and at the same time better yield per wafer.
Featuring, e.g., a new chamber design, the NanoLith 7000 incorporated new technological advances in power design, laser discharge chamber, and wavelength control to enable tight control of exposure dose energy (<±10.3%) and laser wavelength stability (<±10.03 pm). On-Board laser metrology provided pulse-to-pulse data acquisition and feedback control to minimize transient wavelength instabilities, thereby enhancing exposure process latitude and CD control.
However, as even further demands for shrinking bandwidth and increased power moved even beyond the technological advances of applicant's assignees world-class single chamber laser systems, it became clear that something would have to replace the single chamber system. Further optimizing such beam parameters as bandwidth, including keeping it within some specific range, e.g., for OPC reasons, rather than just a not to exceed specification, was becoming impossible at the necessary average power levels. At the time, it was also deemed that increasing repetition rate was not the effective path to take for a number of reasons.
Applicant's assignee's chosen solution was a two chambered laser system comprising a seed laser pulse beam producing laser chamber, e.g., a master oscillator (“MO”), also of the gas discharge excimer variety, seeding another laser chamber with an amplifying lasing medium, also of the same excimer gas discharge variety, acting to amplify the seed beam, a power amplifier (“PA”). Other so-called master oscillator-power amplifier (“MOPA”) laser systems had been known, mostly in the solid state laser art, essentially for boosting power output. applicants' assignee came up with the concept of the utilization of seed laser chamber in which a seed laser was produced, with the view of optimizing that chamber operation for selecting/controlling desirable beam parameters, e.g., bandwidth, beam profile, beam spatial intensity distribution, pulse temporal shape, etc. and then essentially amplifying the pulse with the desirable parameters in an amplifier medium, e.g., the PA. This breakthrough by applicants' assignee was able to meet the then current demands attendant to the continually shrinking node sizes for semiconductor photolithography DUV light sources.
The first of these two chamber laser systems was the XLA-100, providing leading edge optical and power performance in an ultra line narrowed, high power argon fluoride (ArF) production light source. The dual chamber Master Oscillator Power Amplifier (MOPA) architecture developed by applicants' assignee, was capable of 40 W of average output power, double the output power of Cymer's earlier, single chamber-based Nanolith 7000 ArF models, while also meeting even increasingly stringent performance and cost requirements necessary for semiconductor chip production at the <100-nm node. Providing an ultra line-narrowed spectral bandwidth of about 0.25 pm FWHM, and about 0.65 pm E95% integral, the tightest spectral bandwidth performance of any deep-ultraviolet (DUV) production light source up to that time, i.e., about 2003, the XLA 100 provided the light that enabled high contrast imaging for lithography tools with an numerical aperture (NA) up to 0.9.
This was mostly because less energy was wasted in the MO chamber in producing the beam with selected optical parameters, e.g., bandwidth, and the amplifier medium provided plenty of amplification to get an output of the MO at about, e.g., 1 mJ up to a PA output of 10 mJ, 40 watts average output power at 4 kHz operating pulse repetition rate. This allowed, e.g., fewer pulses per exposure window, e.g., enabling the use of less pulses per exposure. The same tight exposure controls were available, i.e., exposure dose (about ±0.3 percent) and wavelength stability (about ±0.025 pm) by providing pulse-to-pulse data acquisition and feedback control to its in-situ metrology system, involving sampling at the outputs of both the MO and PA.
In about the end of 2005 applicants' assignee introduced the XLA-200, a second generation two chamber XLA laser system which became the world's first immersion photolithography enabling gas discharge laser light source even further reduced the ultra line narrowed output at 50% more average output power than the original XLA-100 series. In a quest for smaller feature sizes, new and innovative technologies were needed to meet the mandate of Moore's law and the concomitant ever smaller CD sizes, especially with extreme ultraviolet sources (EUV”) delivery dates being moved out to the end of the first decade of the new millennium or further. The introduction of a fluid of different refraction index than air, e.g., water, to the exposure process, known as Immersion Lithography, was becoming a cost-effective and production-viable technique for extending 193 nm wavelength lithography technologies to meet sub-65 nm process nodes, i.e., ultra-high numerical aperture (NA), immersion-scanner systems.
The XLA 200 met stringent performance and cost requirements necessary for the most sophisticated semiconductor chip production techniques-providing ultra-pure spectral performance of about 0.12 pm FWHM and 0.25 pm E95% integral-to support the ultra-high NA scanner systems required for sub-65 nm exposure, while simultaneously providing high power, (up to 60 W), to support the industry's high productivity needs. Leading edge spectral metrology, used in the XLA series, also enabled monitoring and maintaining the very high spectral purity, including on-board high-accuracy E95% intensity integral bandwidth metrology, e.g., for providing the process control needed for immersion lithography technologies. The XLA 200 was a 193 nm, 4 kHz, 15 mJ, 60 W two-chamber laser system.
Subsequently applicants' assignee introduced in about early 2006 the XLA 300, a 6 kHz 90 W version of the XLA 200. For 193 nm immersion lithography has emerged as the leading for the critical layer processing down to the 32 nm node XLA 300 meet the requirements. Even at the 45 nm node, requirements for critical dimensions, profiles, line edge roughness and overlay requirements of different layers impact design margin and limit yield. High throughput hyper NA (>1.2) exposure tools along with polarized illumination effects and optimized Resolution Enhancement Techniques (RET) will be required for process control, which can be met only by the introduction of applicants assignee's XLA 300 series laser systems. With the k1 physical limit at 0.25 (for memory applications <0.30 is aggressive, logic is usually higher), for 45 nm processing high NA exposure tools and high spectral power (high laser power & high spectral purity) lasers are required. This is what the applicants; assignee's XLA 300 series of lasers currently delivers.
Unfortunately, Moore's law is not done and EUV is still a development project. Therefore addressing even higher power requirements for 193 nm laser light sources is required. Two major obstacles to the typical pulse repetition rate increases evidenced in the advancement of power output in the above noted single chamber laser systems and later two chamber laser systems is the difficulty of getting excimer gas discharge laser system chambers to operate at above 6 kHz and the increased optical damage to certain optical components exposed to the most severe doses of 193 nm light during operation as the pulse repetition rate goes even higher, even with the MOPA architecture. In addition, for various reasons, including the higher pressure operation of the MO's needed, e.g., to extract as much pulse energy as possible out of the line narrowed MO chamber, e.g., around 380 kPa total gas pressure, with, e.g., around a maximum of about 38 kPa of fluorine partial pressure caused conditions advantageous to longer chamber lifetimes to not be attainable, which along with LNM lifetime issues was contributing to the increase in CoC of XLA laser systems.
This latest generation MOPA-based Argon Fluoride light source can provide an ultra-line narrowed bandwidth as low as 0.12 pm FWHM and 0.30 pm 95% energy integral laser light source supporting very high numerical aperture dioptric and catadioptric lens immersion lithography scanners. The XLA 300 introduces an extendable 6 kHz platform to deliver 45 to 90 W of power. Increased repetition rate, along with pulse stretching, minimizes damage to the scanner system optics. State-of-the-art on-board E95% bandwidth metrology and improved bandwidth stability to provide enhanced Critical Dimension control. Longer chamber lifetimes and proven power optics technology extends the lifetime of key laser modules to improve (reduce) CoC (Cost of Consumables), through, e.g., longer chamber lifetimes and proven tower optics technology that extends the lifetime of key laser modules.
In the area of simply generating high average power, e.g., around 100 w and above and up to even 200 w and above, with laser systems operating at not much greater than 6 kHz, the MOPA system, eliminating the line narrowing from the required architecture for the MO still requires new technology.
One possible solution is to use an amplifying medium that comprises a power oscillator. The PA of applicants' assignee is optimized both for amplification and for preservation of the desirable output beam pulse parameters produced in the MO with optimized, e.g., line narrowing. An amplifier medium that is also an oscillator, a power oscillator (“PO”), has been proposed and used by applicants' assignee's competitor GigaPhoton, as evidenced in U.S. Pat. No. 6,721,344, entitled INJECTION LOCKING TYPE OR MOPA TYPE OF LASER DEVICE, issued on Apr. 13, 2004 to Nakao et al; U.S. Pat. No. 6,741,627, entitled PHOTOLITHOGRAPHIC MOLECULAR FLUORINE LASER SYSTEM, issued on May 25, 2004 to Kitatochi et al, and U.S. Pat. No. 6,839,373, entitled ULTRA-NARROWBAND FLUORINE LASER APPARATUS, issued on Jan. 4, 2005 to Takehisha et al.
Unfortunately the use of an oscillator such as with front and rear reflecting mirrors (include a partially reflecting output coupler, and input coupling, e.g., through an aperture in one of the or through, e.g., a 95% reflective rear reflector) has a number of drawbacks. The input coupling from the MO to the amplifier medium is very energy loss-prone. In the amplifier medium with such an oscillator cavity optimized beam parameters selected, e.g., in the MO chamber, may be denigrated in such an oscillator used as an amplifying medium. An unacceptable level of ASE may be produced.
Applicant's propose an architecture that can preserve the optimized beam parameters developed in an MO chamber almost to the same degree as applicants' assignee's present XLA XXX systems, while producing much more efficient amplification from the amplification medium, e.g., to give current levels of output average power with strikingly reduced output pulse energy from the MO (seed laser) resulting in, e.g., a much lower CoC for the MO. Further, applicants believe that according to aspects of embodiments of the subject matter disclosed, e.g., pulse-to-pulse stability or a number of laser output parameters can also be greatly improved.
Increased Wafer Throughput & Productivity maintaining the advancing requirements for Deep Ultra Violet lithography in mass production, and increasing importance on the economics of the laser use is satisfied in part by increasing the laser's repetition rate to 6 kHz and output power to 90 W. Resolution and critical dimension (CD) control in advanced lithography, at the 193 nm, requires a narrow spectral bandwidth, e.g., because all lens materials have some degree of chromatic aberration, necessitating a narrow bandwidth laser to reduce the wavelength variation in the light source, thereby diminishing the impact of chromatic aberration. Very narrow bandwidth can improve the ultimate resolution of the system, or, alternatively can give lens designers more focal latitude. Expensive calcium fluoride optics suffer less chromatic aberration at 193 nm than fused silica does. Narrow bandwidth lasers reduce the need for calcium fluoride optics in 193 nm exposure systems. Spectral engineering, e.g., for critical dimension (CD) control, e.g., driven by more aggressive use of optical proximity correction and higher NA lenses increased the sensitivity to BW and BW changes, including not just bandwidth specification of not to exceed, but bandwidth specification of within a relatively narrow range between a high (the formerly not to exceed type of limit) and a low. Stable BW is even more important for ArF than it has been for KrF. Even a very low BW can yield poor CD if exhibiting significant variations underneath the upper limit. Thus, both BW metrology and BW stabilization are critical technologies for good CD control.
A 6 kHz Repetition Rate results also in improved dose performance to minimize CD variation at the 45 nm node, which can reduced dose quantization errors, e.g., that occur when the exposure slit does not capture all pulses in the dose. In addition, dose errors due to laser beam dynamics can cause imperfections of the exposure slit profile. A newly designed LNM for the XLA 300, e.g., uses a higher resolution dispersive element and an improved wavelength control actuation mechanism, which improved LNM in combination with applicant's assignee's Reduced Acoustic Power (RAP) chamber provides excellent stability of bandwidth.
Other problems exist in such arrangements, e.g., ASE production can be significant enough, e.g., in the power amplification stage to cause downstream problems in the line narrowed versions, because the ASE is out of band. The ASE may also cause problems in the, e.g., broadband, e.g., LTPS versions, since the beam treatment optics, e.g., to produce an elongated and very thin, e.g., 10μ or so wide, beam may be sensitive to light well out of the broadband normally produced by excimer lasing in the amplification stage. In addition, ASE can rob gain medium and thus lower the available in-band or otherwise useable output of the amplification stage.
Buczek, et al, CO2 Regenerative Ring Power Amplifiers, J. App. Phys., Vol. 42, No. 8 (July 1971) relates to a unidirectional regenerative ring CO2 laser with above stable (conditionally stable) operation and discusses the role of gain saturation on CO2 laser performance. Nabors, et al, Injection locking of a 13-W Nd:YAG ring laser, Optics Ltrs., vol. 14, No 21 (November 1989) relates to a lamp-pumped solid-state CW ring laser injection locked by a diode-pumped solid state Nd:YAG master oscillator. The seed is input coupled into the ring laser by a half-wave plate, a Faraday rotator and a thin film polarizer forming an optical diode between the seed laser and the amplifier. Pacala, et al., A wavelength scannable XeCl oscillator—ring amplifier laser system, App. Phys. Ltrs., Vol. 40, No. 1 (January 1982); relates to a single pass excimer (XeCl) laser system seeded by a line narrowed XeCl oscillator. U.S. Pat. No. 3,530,388, issued to Buerra, et al. on Sep. 22, 1970, entitled LIGHT AMPLIFIER SYSTEM, relates to an oscillator laser seeding two single pass ring lasers in series with beam splitter input coupling to each. U.S. Pat. No. 3,566,128, issued to Amaud on Feb. 23, 1971, entitled OPTICAL COMMUNICATION ARRANGEMENT UTILIZING A MULTIMODE OPTICAL REGENERATIVE AMPLIFIER FOR PILOT FREQUENCY AMPLIFICATION, relates to an optical communication system: with a ring amplifier. U.S. Pat. No. 3,646,468, issued to Buczek, et al. on Feb. 29, 1972 relates to a laser system with a low power oscillator, a high power oscillator and a resonance adjustment means. U.S. Pat. No. 3,646,469, issued to Buczek, et al. on Feb. 29, 1097, entitled TRAVELLING WAVE REGENERATIVE LASER AMPLIFIER, relates to a laser system like that of the '468 Buczek patent with a means for locking the resonant frequency of the amplifier to frequency of the output of the oscillator. U.S. Pat. No. 3,969,685, issued to Chenausky on Jul. 13, 1976, entitled ENHANCED RADIATION COUPLING FROM UNSTABLE LASER RESONATORS relates to coupling energy from a gain medium in an unstable resonator to provide a large fraction of the energy in the central lobe of the far field. U.S. Pat. No. 4,107,628, issued tot Hill, et al., on Aug. 15, 1978, entitled CW BRILLOUIN RING LASER, relates to a Brillouin scattering ring laser, with an acousto-optical element modulating the scattering frequency. U.S. Pat. No. 4,135,787, issued to McLafferty on Jan. 23, 1979, entitled UNSTABLE RING RESONATOR WITH CYLINDRICAL MIRRORS, relates to an unstable ring resonator with intermediate spatial filters. U.S. Pat. No. 4,229,106, issued to Domschner on Oct. 21, 1980, entitled ELECTROMAGNETIC WAVE RING GENERATOR, relates to a ring laser resonator with a means to spatially rotate the electronic field distribution of laser waves resonant therein, e.g., to enable the waves to resonate with opposite polarization. U.S. Pat. No. 4,239,341 issued to Carson on Dec. 16, 1980, entitled UNSTABLE OPTICAL RESONATORS WITH TILTED SPHERICAL MIRRORS, relates to the use of tilted spherical mirrors in an unstable resonator to achieve asymmetric magnification to get “simultaneous confocality” and obviate the need for non-spherical mirrors. U.S. Pat. No. 4,247,831 issued to Lindop on Jan. 27, 1981, entitled RING LASERS, relates to a resonant cavity with at least 1 parallel sided isotropic refracting devices, e.g., prisms, with parallel sides at an oblique angle to part of light path that intersects said sides, along with a means to apply oscillating translational motion to said refracting devices. U.S. Pat. No. 4,268,800, issued to Johnston et al. on May 19, 1981, entitled, VERTEX-MOUNTED TIPPING BREWSTER PLATE FOR A RING LASER, relates to a tipping Brewster plate to fine tune a ring laser located close to a flat rear mirror A acting as one of the reflecting optics of the ring laser cavity. U.S. Pat. No. 4,499,582, entitled RING LASER, issued to Karning et al. on Feb. 5, 1980, relates to a ring laser system with a folded path pat two separate pairs of electrodes with a partially reflective input coupler at a given wavelength. U.S. Pat. No. 5,097,478, issued to Verdiel, et al. on Mar. 17, 1992, entitled RING CAVITY LASER DEVICE, relates to a ring cavity which uses a beam from a master laser to control or lock the operation of a slave laser located in the ring cavity. It uses a non-linear medium in the cavity to avoid the need of insulators, e.g., for stabilizing to suppress oscillations, e.g., as discussed in Col 4 lines 9-18. Nabekawa et al., 50-W average power, 200-Hz repetition rate, 480-fs KrF excimer laser with gated gain amplification, CLEO (2001), p. 96, e.g., as discussed with respect to FIG. 1, relates to a multipass amplifier laser having a solid state seed that is frequency multiplied to get to about 248 nm for KrF excimer amplification. U.S. Pat. No. 6,373,869, issued to Jacob on Apr. 16, 2002, entitled SYSTEM AND METHOD FOR GENERATING COHERENT RADIATION AT ULTRAVIOLET WAVELENGTHS, relates to using an Nd:YAG source plus an optical parametric oscillator and a frequency doubler and mixer to provide the seed to a multipass KrF amplifier. U.S. Pat. No. 6,901,084, issued to Pask on May 31, 2005, entitled STABLE SOLID STATE RAMAN LASER AND A METHOD OF OPERATING SAME, relates to a solid-state laser system with a Raman scattering mechanism in the laser system oscillator cavity to frequency shift the output wavelength. U.S. Pat. No. 6,940,880, issued to Butterworth, et al. on Sep. 6, 2005, entitled OPTICALLY PUMPED SEMICONDUCTOR LASER, relates to a optically pumped semiconductor laser resonance cavities forming part of a ring resonator, e.g., with a non linear crystal located in the ring, including, as discussed, e.g., with respect to FIGS. 1, 2, 3, 5 & 6, having a bow-tie configuration. United States Published Patent Application No. 2004/0202220, published on Oct. 14, 2004, with inventors Hua et al, entitled MASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, relates to an excimer laser system, e.g., with in a MOPA configuration, with a set of reflective optics to redirect at least a portion of the oscillator beam transmitted through the PA back thru PA ion the opposite direction. United States Published Patent Application No. 2005/0002425, published on Jan. 1, 2003, with inventors Govorkov et al, entitled MASTER-OSCILLATOR POWER-AMPLIFIER (MOPA) EXCIMER OR MOLECULAR FLUORINE LASER SYSTEM WITH LONG OPTICS LIFETIME, relates to, e.g., a MOPA with a pulse extender and using a beamsplitting prism in the pulse extender, a housing enclosing the (MO+PA) and reflective optics, with the pulse extender mounted thereon, and reflective optics forming a delay line around the PA. United States Published application No. 2006/0007978, published on Jan. 12, 2006, with inventors Govokov, et al., entitled BANDWIDTH-LIMITED AND LONG PULSE MASTER OSCILLATOR POWER OSCILLATOR LASER SYSTEM, relates to a ring oscillator with a prism to restrict bandwidth within the oscillator.
U.S. Pat. No. 6,590,922 issued to Onkels et al. on Jul. 8, 2003, entitled INJECTION SEEDED F2 LASER WITH LINE SELECTION AND DISCRIMINATION discloses reverse injection of and F2 laser undesired radiation centered around one wavelength through a single pass power amplifier to selectively amplify a desired portion of the F2 spectrum for line selection of the desired portion of the F2 spectrum in a molecular fluorine gas discharge laser. in F2 laser.
U.S. Pat. No. 6,904,073 issued to Yager, et al. on Jun. 7, 2005, entitled HIGH POWER DEEP ULTRAVIOLET LASER WITH LONG LIFE OPTICS, discloses intracavity fluorine containing crystal optics exposed to lasing gas mixtures containing fluorine for protection of the optic.
Published International application WO 97/08792, published on Mar. 6, 1997 discloses an amplifier with an intracavity optical system that has an optical path that passes each pass of a sixteen pass through the same intersection point at which is directed a pumping source to amplify the light passing through the intersection point.
R. Paschotta, Regenerative amplifiers, found at http: //www.rp-photonics.com/regenerative_amplifiers.html (2006) discusses the fact that a regenerative amplifier, may be considered to be an optical amplifier with a laser cavity in which pulses do a certain number of round trips, e.g., in order to achieve strong amplification of short optical pulses. Multiple passes through the gain medium, e.g., a solid state or gaseous lasing medium may be achieved, e.g., by placing the gain medium in an optical cavity, together with an optical switch, e.g., an electro-optic modulator and/or a polarizer. The gain medium may be pumped for some time, so that it accumulates some energy after which, an initial pulse may be injected into the cavity through a port which is opened for a short time (shorter than the round-trip time), e.g., with the electro-optic (or sometimes acousto-optic) switch. Thereafter the pulse can undergo many (possibly hundreds) of cavity round trips, being amplified to a high energy level, often referred to as oscillation. The electro-optic switch can then be used again to release the pulse from the cavity. Alternatively, the number of oscillations may be determined by using a partially reflective output coupler that reflects some portion, e.g., around 10%-20% of the light generated in the cavity back into the cavity until the amount of light generated by stimulated emission in the lasing medium is such that a useful pulse of energy passes through the output coupler during each respective initiation and maintenance of an excited medium, e.g., in an electrically pumped gas discharge pulsed laser system, the gas discharge between the electrodes caused by placing a voltage across the electrodes at the desired pulse repetition rate. Uppal et al, Performance of a general asymmetric Nd:glass ring laser, Applied Optics, Vol. 25, No. 1 (January 1986) discusses an Nd:glass ring laser. Fork, et al. Amplification of femptosecond optical pulses using a double confocal resonator, Optical Letters, Vol. 14, No. 19 (October 1989) discloses a seed laser/power amplifier system with multiple passes through a gain medium in a ring configuration, which Fork et al. indicates can be “converted into a closed regenerative multi pass amplifier by small reorientations of two of the four mirrors that compose the resonator [and providing] additional means . . . for introducing and extracting the pulse from the closed regenerator. This reference refers to the open-ended amplifier portion with fixed number of passes through the amplifier portion (fixed by the optics an, e.g., how long it takes for the beam to walk off of the lens and exit the amplifier portion as a “resonator”. As used herein the term resonator and other related terms, e.g., cavity, oscillation, output coupler are used to refer, specifically to either a master oscillator or amplifier portion, the power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, e.g., the size of the cavity and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, e.g., a one pass, two pass, etc. power amplifier, or six or so times in the embodiment disclosed in Fork, et al. Mitsubishi published Japanese Patent Application Ser. No. JP11-025890, filed on Feb. 3, 1999, published on Aug. 11, 2000, Publication No. 2000223408, entitled SEMICONDUCTOR MANUFACTURING DEVICE, AND MANUFACTURING OF SEMICONDUCTOR DEVICE, disclosed a solid state seed laser and an injection locked power amplifier with a phase delay homogenizer, e.g., a grism or grism-like optic between the master oscillator and amplifier. United states Published application 20060171439, published on Aug. 3, 2006, entitled MASTER OSCILLATOR-POWER AMPLIFIER EXCIMER LASER SYSTEM, a divisional of an earlier published application 20040202220, discloses as master oscillator/power amplifier laser system with an optical delay path intermediate the master oscillator and power amplifier which creates extended pulses from the input pulses with overlapping daughter pulses.
Partlo et al, Diffuser speckle model: application to multiple moving diffusers, discusses aspects of speckle reduction. U.S. Pat. No. 5,233,460, entitled METHOD AND MEANS FOR REDUCING SPECKLE IN COHERENT LASER PULSES, issued to Partlo et al. on Aug. 3, 1993 discusses misaligned optical delay paths for coherence busting on the output of gas discharge laser systems such as excimer laser systems.
The power efficiency of a regenerative amplifier, e.g., using a switching element, can be severely reduced by the effect of intracavity losses (particularly in the electro-optic switch). Also, the reflectivity of a partially reflective output coupler can affect both intracavity losses and the duration of the output pulse, etc. The sensitivity to such losses can be particularly high in cases with low gain, because this increases the number of required cavity round trips to achieve a certain overall amplification factor. A possible alternative to a regenerative amplifier is a multipass amplifier, such as those used in applicants' assignee's XLA model laser systems mentioned above, where multiple passes (with, e.g., a slightly different propagation direction on each pass) can be arranged with a set of mirrors. This approach does not require a fast modulator, but becomes complicated (and hard to align) if the number of passes through the gain medium is high.
An output coupler is generally understood in the art to mean a partially reflective optic that provides feedback into the oscillation cavity of the laser and also passes energy out of the resonance cavity of the laser.
In regard to the need for improvement of Cost Of Consumables, e.g., for ArF excimer lasers, e.g., for photolithography light source use, KrF CoC has long been dominated by chamber lifetime, e.g., due to the robustness of the optics at the higher 248 nm wavelength for KrF. Recent advances in Cymer ArF optical components and designs have led to significant increases in ArF optical lifetimes, e.g., ArF grating life improvements developed for the Cymer NL-7000A, Low intensity on LNMs, e.g., in two stage XLA systems. ArF etalon material improvements have contributed to longer life for ArF wavemeters, stabilization modules, LAMs, SAMs, and BAMs. In addition KrF chamber lifetime has been significantly increased with Cymer ELS-7000 and ELS-7010 products, e.g., through the use of proprietary electrode technology. However, longer life electrode technology requires specific operating parameters, such as are met in ELS-7000 and ELS-7010 KrF chambers, XLA-200 and XLA-300 PA chambers. These parameters, however, are not able to be utilized, e.g., in any of Cymer's ArF XLA MO chambers because of the overall output power requirements of the system. Applicants propose ways to alleviate this detriment to cost of consumables in, e.g., the ArF dual chamber master oscillator/amplifier products, used, e.g., for integrated circuit manufacturing photolithography.
As used herein the term resonator and other related terms, e.g., cavity, oscillation, output coupler are used to refer, specifically to either a master oscillator or amplifier portion, a power oscillator, as lasing that occurs by oscillation within the cavity until sufficient pulse intensity exists for a useful pulse to emerge from the partially reflective output coupler as a laser output pulse. This depends on the optical properties of the laser cavity, e.g., the size of the cavity and the reflectivity of the output coupler and not simply on the number of reflections that direct the seed laser input through the gain medium a fixed number of times, e.g., a one pass, two pass, etc. power amplifier.